Formation of Carbon Nanotube Bucky Paper and Feasibility Study for

Article pubs.acs.org/JPCC

Formation of Carbon Nanotube Bucky Paper and Feasibility Study for Filtration at the Nano and Molecular Scale Soumyendu Roy,†,∥ Vishal Jain,†,∥,⊥ Reeti Bajpai,†,∥ Pintu Ghosh,†,§ A. S. Pente,‡,§ B. P. Singh,†,§ and D. S. Misra*,†,∥ †

Department of Physics, Indian Institute of Technology Bombay, Mumbai 400076, India Process Control Laboratories, Waste Management Division, Nuclear Recycle Group, Bhabha Atomic Research Centre, Mumbai-85, India

‡

S Supporting Information *

ABSTRACT: Presence of abundant pores and highly entangled nanotubes make bucky paper (BP) a natural candidate for filtration related applications. Both single- and multiwall carbon nanotube (SWNT and MWNT) BPs were fabricated via self-assembly. Average diameter of the pores on the surface of MWNT BP appeared to be 33 ± 15 nm. However, due to the high tortuousity of BP, the cutoff size, estimated by filtration of colloidal dispersions of Au and CdS nanoparticles of different diameters, turned out to be 4−5 nm. The particle sizes were verified using microscopic and spectroscopic methods. The flux of Au nanoparticle solutions through BP was about 1000 L h−1 m−2 bar−1. Highly flexible and robust SWNT BPs were prepared very easily, even without proper dispersion. It was found that the crystallinity and purity of the SWNTs had a bigger role than the quality of dispersion in determining the final mechanical strength of BP. The density of SWNT BP was estimated to be 1.3 ± 0.3 g/cm3 after correcting for the weights of the impurities. This is among the highest ever reported. The average wall to wall separation of adjacent tubes was estimated to be 0.35 ± 0.2 nm, which is very close to the ideal value. The SWNT BP was also found to be impervious to liquids such as water, n-hexane, acetone, and isopropyl alcohol, indirectly verifying its close knit structure and small pore size. This is indicative of its possible use as a molecular sieve membrane.

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storage, solar cells, Li ion batteries, heat sink, and so forth.1−8 BP has a laminar structure with networks of intertwined CNTs held together by van der Waals forces at tube−tube junctions. The tubes are generally oriented randomly in the plane of the BP unless special techniques such as application of electric and magnetic fields9 are used to align them preferentially along a particular direction. The material is dominated by pores through which liquids and gases can permeate. Molecular dynamics simulations have predicted higher fluid fluxes through CNTs than those allowed by Knudsen flow. This enhancement is generally attributed to the atomic smoothness of the nanotube surface and to molecular ordering phenomena that occur inside the tubes because of their nanometer scale dimension.10 Membranes consisting of vertically aligned CNTs embedded in a matrix of polystyrene11 and silicon nitride12 have been prepared. By blocking the interstitial regions, the fluid is forced to flow through the hollow interior of the CNTs. This allows very fine control of the pore diameter distribution. However, the fabrication technique of such membranes is very cumbersome, and their porosity is low. For industrial and commercial applications such as air and water purification,

INTRODUCTION In the ever-expanding field of nanoscience, the carbon nanotube (CNT) is currently one of the most researched and explored materials. Though CNTs are feather light in their construction, these are much stronger in nature and are used in electronics, optics, materials science, aerospace, automobile construction, and in a wide variety of applications. At the molecular level, it has a pseudo-one-dimensional (1D) structure with a single or multiple coaxial cylinders of sp2 hybridized carbon atoms arranged in a honeycomb lattice, which are called single wall or multiwall CNTs (SWNT and MWNT) accordingly. Macroscopic freestanding films of CNTs are called bucky papers (BPs). These are formed via self-assembly of CNTs. CNTs dispersed in an appropriate solvent are filtered through a membrane leaving a mat of CNTs on it, which can be recovered on drying. The most commonly used solvents are isopropyl alcohol (IPA), N-methylpyrrolidone (NMP), and N,N-dimethylformamide (DMF). CNTs can also be dispersed in water by means of suitable surfactants such as sodium dodecyl sulfate (SDS).1 Functionalization of nanotubes that facilitates cross linking and improves the mechanical strength of BP has also been used. BPs and similar films of CNTs have yielded promising results in various applications such as actuators, fuel cell catalyst supports, electrodes in supercapacitors, field emitters, nanoscale generators, hydrogen gas © 2012 American Chemical Society

Received: June 11, 2012 Revised: August 13, 2012 Published: August 16, 2012 19025

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CH4 and H2 gases (in the ratio 1:5 by volume) was blown over heated catalyst kept at a temperature of 960−970 °C under atmospheric pressure. The product was stirred in 11 M HNO3 solution for about 1.5 h. The acid dissolved the catalyst material as well as dispersed the nanotubes. The dispersion was then filtered in the same manner as the MWNTs above. However, during the filtration, a continuous supply of deionized water was maintained until the pH was ∼7. After drying, the SWNT BPs were separated from the PVDF filter by dipping in N,Ndimethyl acetamide (a mild solvent for PVDF). Although the results reported here were obtained with SWNTs grown using a Fe-based catalyst, similar phenomena were also observed with tubes grown by a Co-based catalyst (Co/Mo/MgO = 1:0.5:300). Catalyst containing higher proportion of Fe (Fe/ Mo/MgO ≈ 1:0.5:30 to 1:0.5:60) produced higher amounts of SWNTs per unit mass of the catalyst. However the amount of impurities also increased. NP Synthesis and Filtration. Free-standing Au NPs were synthesized using the Turkevich method, involving single-phase water-based reduction of auric chloride (HAuCl4) by sodium citrate.29 Almost spherical particles were produced whose size depended on the relative amount of the two reacting components. The smallest Au NPs had a mean diameter of 14.7 ± 0.7 nm and were grown using a citrate/ auric chloride molar ratio of 3:1. CdS NPs were synthesized by irradiating solutions of CdSO4 and 2-mercaptoethanol with the Co60 γ rays source (2.65 kGy/h), at a pH of 7.5 (buffer: sodium phosphate).30,31 The solution has been exposed to γ rays to an integrated dose of 7.5 kGy. The colloidal dispersions of Au and CdS NPs were filtered through the MWNT BP (thickness 330 ± 5 μm). These experiments were conducted in a simple vacuum filtration setup (unstirred dead-end configuration), similar to the one used in the formation of BP. Vacuum was applied to the permeate side to maintain a pressure difference of 160 Torr across the filter. Epoxy was used as a sealant to make sure that the only pathway available to the solution is through the BP. SWNT BP was also used in the same configuration but with a pressure difference of 500−600 Torr while testing for the permeation of liquids. Instrumental Techniques Used for Characterizations. Surface morphology of SWNTs and MWNTs (before and after filtration) was studied using JEOL, JSM-6400, and JEOL, JSM7600F, scanning electron microscopes (SEMs). The latter one had an Oxford Instrument, INCA wave spectrometer fitted within it that was used for energy dispersive X-ray spectroscopy (EDS). The high-resolution transmission electron microscope (HRTEM) and the UV−visible spectrometer used were Jeol, JEM 2100F, and Perkin−Elmer Lambda 950, respectively. Raman scattering, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA) of the BPs were conducted using Horiba Jobin Yvon, HR 800, Thermo vg Scientific, multilab 2000, and Perkin−Elmer diamond TG/ DTA, respectively.

petroleum processing, and so forth, factors such as cost, ease of fabrication, operational simplicity, and scalability are more important. BP is highly suited for such applications. Although BP has been known to the scientific community for a long time, its filtration-based applications have started appearing only recently.13 Ajayan et al. reported the first use of a simple macroscopic structure of as synthesized MWNTs as a filter.14 They were able to separate certain heavy hydrocarbons from petroleum as well as Escherichia coli bacteria and poliovirus (25 nm) from water. Recently, membranes for size selective separation of nanoparticles (NPs) were prepared using carbonaceous nanofibers (cutoff is 25 nm).15 Li et al. used spongelike agglomerates of MWNTs to filter NPs and dye molecules.16 In 2010, a thermally driven direct contact membrane desalination technique of seawater using hydrophobic BP was demonstrated.17 Ajayan et al. had recently developed three-dimensional (3D) scaffolds by growing MWNTs inside micromachined Si/SiO2 templates and used these to filter submicrometer aerosol particles and as support membranes for a gas-phase heterogeneous catalysis.18 In the present study, MWNT BP was used to filter particles down to a few nanometers. Using Au and CdS NPs of different diameters, the cutoff size of BP was estimated to be 4−5 nm. Filtration was performed under a transmembrane pressure of 160 Torr, and the flux attained was ≈1000 L h−1 m−2 bar−1. SWNT BPs were also prepared. These were found to be very strong and flexible unlike the MWNT BPs that were rigid and brittle. The enhanced strength and flexibility may be attributed to the high density and close proximity of the tubes in SWNT BP. The SWNT BPs could be made thin (5−10 μm) whereas the MWNT BPs had to be necessarily thick (a few hundred μm to a mm). To the best of our knowledge, the density of the SWNT BPs reported here (1.3 ± 0.3 g/cm3) is among the highest9,19−23 and is very close to the value expected for a hexagonal crystalline arrangement of SWNTs (1.34 g/cm3). The average intertubular separation turns out to be 0.35 ± 0.2 nm. For this reason, these BPs were found to be impervious to common liquids, even under applied pressure. This suggests that the accessible pores are small enough to make SWNT BPs a prospective candidate for membrane-based technologies used for separating gases by molecular sieving.24 This method has already been demonstrated using zeolite,25 ceramics,26 and nanoporous carbon membranes.27

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EXPERIMENTAL METHODS MWNT and BP Synthesis. Films of vertically aligned MWNT were synthesized by thermal chemical vapor deposition (CVD).28 Substrate was a p-type Si (100) wafer with a thermally grown 50 nm layer of SiO2 on it. Ferrocene dissolved in toluene (density = 0.02 g/mL) was evaporated and transported into the hot zone of the tube furnace by hydrogen gas flow of 70 sccm. The synthesis time for CNTs was about 45 min, and the deposition temperature was 800−810 °C. The nanotubes were scrapped from the wafer, dispersed in isopropyl alcohol, and filtered through a polyvinylidene fluoride (PVDF) membrane (0.22 μm pore size). A constant pressure difference of about 600−700 Torr was maintained during the filtration. After drying, the BP could be peeled off easily from the filter. Both MWNT and SWNT BPs were 1.7−1.9 cm in diameter. SWNT and BP Synthesis. SWNTs were grown by CVD of CH4 in the presence of H2 gas. The catalyst consisted of a solid solution of Fe and Mo in MgO (Fe/Mo/MgO = 1:0.5:200 by weight) prepared by combustion synthesis.7 The mixture of

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RESULTS AND DISCUSSION Figure 1a shows the Raman spectrum of the MWNT samples. It has an ID/IG ratio of 0.71, which is typical for this class of carbon nanomaterials. The EDS spectrum (shown in Figure 1b) revealed that about 94% (by weight) of these material is C and only 4% is Fe (from the catalyst), with the remaining being O. Because of the low amount of impurity, MWNTs were used as is, without any purification for making BPs. XPS spectra showing the oxygen content in MWNT BP can be seen in the 19026

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520 nm in UV−visible absorption spectra (Figure 2c) of the solution after filtration. The same results were obtained with Au particles of larger sizes. CdS NPs having a wide size distribution of 2−10 nm and an average diameter of 4.1 ± 2.1 nm (as observed in HRTEM (see Figure 2e) were filtered next. Some of the particles were blocked while some seeped through. The average diameter of the CdS NPs in the permeate was 2.9 ± 0.5 nm, and the largest among these particles had dimensions between 4 and 4.5 nm (see Figure 2f). These microscopic observations were also validated using the UV−visible absorption spectra shown in Figure 2d. From the band gap absorption edge in the spectra and using the effective mass approximation formula derived by Brus32 (see the Supporting Information), one can get an approximate value of the mean diameter of the NPs. These values for the feed and the permeate solutions turn out to be 3.8 and 2.4 nm, respectively, which corroborate well with the average values obtained from HRTEM images. Hence, it can be safely assumed that the cutoff size of BP was 4−5 nm. This value is much lower than those reported with macroscopic cylindrical filters made from vertically aligned MWNTs,14 spongelike material made from CNTs16 (this one operates mainly through physisorption rather than size exclusion), membranes made from materials such as carbonaceous nanofibers,15 polycarbonate,33 and polymerbased filters with engineered nanopores in them,34 certain cellulose,33,35 and ceramic nanofibers.33,36 Some applications of CNT-based systems for air purification via removal of aerosols have also been reported. These generally have a much larger cutoff size of about 300 nm.18 During filtration of the colloidal solutions of the NPs (both Au and CdS), the pressure drop across the BP was 160 Torr (21.33 KPa), and the flux was

Figure 1. (a) Raman spectrum of MWNTs. Inset is a photograph of the disklike BP formed from MWNTs. (b) A representative EDS spectrum of MWNT BP. The peaks for O and Fe have been magnified in the inset. The amount of impurity is about 6% by weight, which is very low.

Supporting Information. The diameter and the number of walls of the nanotubes both varied widely with mean values 30 ± 8 nm and 21 ± 9, respectively. Figure 2 shows the results of the filtration experiments using the MWNT BP. From the SEM images as shown in Figure 2a, the apparent size of the interstitial pores visible on the BP surface was found to be 33 ± 15 nm. In spite of such large pores, BP is able to intercept and remove much smaller particles because of the highly tortuous paths that the liquid has to take while flowing through the cross-linked three-dimensional structure of BP. In Figure 2b Au NPs can be seen stuck on the BP after filtration. These particles had an average diameter of 14.7 ± 0.7 nm, as evaluated using HRTEM images (see the inset in Figure 2b). 100% removal of these particles from a colloidal solution of 0.25 mM concentration could be achieved. This was confirmed by the complete disappearance of the characteristic plasmon peak at

Figure 2. (a) Surface morphology of MWNT BP as seen in a SEM. Scale bar is 100 nm. (b) Surface of the BP after Au NP filtration. Scale bar is again 100 nm, and the inset is an HRTEM image showing the Au NPs. (c) UV−visible absorption spectra of the colloidal solution of Au NPs before and after filtration through MWNT BP. The plasmon peak at 520 nm completely disappears after filtration, and the solution changes from reddish to colorless as shown in the inset. (d) UV−visible absorption spectra of CdS NP solutions before and after filtration. The absorption edge shifts from about 458 to 395 nm indicating that only the smaller NPs are able to flow through the BP. (e, f) The HRTEM images of the CdS NPs in the feed and the permeate, respectively. The two images have been scaled proportionally so that they can be compared directly. Scale bars in both measure 10 nm. 19027

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Figure 3. (a) Typical Raman spectrum of SWNT BP showing a small defect or D band around 1333 cm−1. Inset is an HRTEM image of a SWNT bundle. (b) SEM micrograph of the BP surface. Although surface undulations are visible, the individual SWNT bundles that make up the BP cannot be seen because of the high density. However, on damaging the surface or tearing the BP, the SWNT bundles start to emerge as shown in the inset. Scale bar of the inset is 500 nm. (c) The picture shows a SWNT dispersion being filtered to make BP. The nanotubes are clearly in an agglomerated state. (d) A normal 8 μm thick SWNT BP. It is highly flexible and robust and can be easily rolled and unrolled without causing any visible damage. (e, f) SWNT BP made by slight variations in the synthesis techniques as described in the main text. The BP in (e) still has the flexibility but has lost the strength and luster of (d). It has a highly wrinkled surface. The BP in (f) is thick and brittle like the MWNT BPs shown in Figure 1.

Figure 4. (a) Typical EDS spectrum of SWNT BP obtained from an area of about 3500 μm2. It gives a composition of 6.7% Fe, 15.3% O (by weight), and the rest C. There are trace amounts of Mo and Mg as shown in the magnified inset. (b) TGA of BP confirms that 6.5% of it is metal. It does not show any signature of significant amounts of amorphous carbon or MWNTs. (c) Deconvulated C1s peak in the XPS spectrum of SWNT BP confirms the presence of O-containing functional groups with the hydroxyl group being the most predominant.

found to be approximately 1000 L h−1 m−2 bar−1. Although this is lower than some of the aforementioned membranes,14,15,35 it exceeds many polycarbonate, polymeric,34 and ceramic nanofilters.36 Such a trade-off between flux and pore size is expected. Membranes containing CNTs embedded in nonporous matrix that allow fluid flow only through the internal pores of the CNTs have similar cutoffs. However, their flux is significantly lower due to their low porosity.12,37 The filtration performance of these BPs are dependent on a multitude of factors such as thickness of the BP, length, diameter and purity of the tubes, degree of entanglement, and so forth. The filter developed by Ajayan et al. has thickness of the same order as the BPs used here; however, in their case, the MWNTs are aligned resulting in lesser entanglement and hence higher cutoff.14 In membranes made from carbon nanofibers,15 it was observed that the cutoff size depended heavily on the diameter of the fibers. Further studies are required to probe such dependencies in CNT BPs. Apart from the applications mentioned above, other exotic applications using functionalized tubes and application of electric field during filtration are also possible.38−40 CNTs have been shown to be cytotoxic toward microorganisms.41 This could provide a cost-effective way to not only remove pathogens from water but also to neutralize them. The Raman spectrum of SWNT BP is shown in Figure 3a. The most important feature is the tiny, almost nonexistent D band, indicating the defect free nature on the tubes.42 Figure 3b shows an SEM image of the surface of SWNT BP. It is

interesting to note that, even at high magnification, individual bundles cannot be resolved. Images published in literature show BP made of SWNT bundles that appear separate, curled up, and interlocked with one another.19,20,22,23,43 In our case, the bundles are visible only if we damage or tear the BP as shown in the inset of Figure 3b. This is perhaps due to the close proximity of the tubes in BP. During the experiments, it was also observed that a perfect dispersion of SWNTs is not required to produce a dense and strong BP. The HNO3 dispersion used for making BP starts to agglomerate within a few minutes even while the filtration is still going on (see Figure 3c). It seems that the long held belief that a good dispersion is a necessary prerequisite for forming BP is misguided. However, in making devices that depend on the properties of individual tubes, a proper dispersion is of utmost importance. The capillary forces that come into play as the solvent evaporates bring the nanotubes very close together. A similar collapse of vertically aligned CNT forest into a dense solid material under the effect of evaporating solvent has recently been observed.19,23 Additionally the defect free SWNT surfaces exert stronger van der Waals forces on one another facilitating the self-assembly process. Stirring in HNO3 solution for about 24 h (as opposed to the 1.5 h duration used here) produces better dispersion but more defects in NTs (see the Supporting Information) resulting in poorer BP as shown in Figure 3d, e. By increasing the concentration of Fe−Mo bimetal catalyst in the MgO matrix, the yield of SWNTs during the CVD process can be increased. However, the amount of 19028

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Table 1. Comparison of SWNT BP Density with Values Published in Literature density (g/cm3)

ref

purification

diameter (nm)

0.57

19

[a]

2.8

0.71 0.9

20 9

[a] yes

1.36 1.3

0.8−1.2 1.3−1.5 1.3 ± 0.3

21, 43 22 this paper

yes [a] Yes

1.2−1.4 1.4 1.35

details aggregates of SWNTs formed from vertically aligned tubes by solvent evaporation; ideal density for 2.8 nm tubes is 0.78 g/cm3 BP is permeable to liquids like methanol BP consisted of tubes aligned along a preferred direction under the influence of strong magnetic fields (7 and 26 T) density decreases as thickness is increased from 200 nm to ∼10 μm, which is unexpected strong SWNT fibers, impurities visible in microscope weight of residual impurities was taken into account while calculating density unlike the studies mentioned above

long as 12−14 h. To rule out the effect of hydrophobicity of the CNTs, organic liquids such as n-hexane (0.62 nm), isopropyl alcohol (0.52 nm), and acetone (0.51 nm) were also tested. All showed the same behavior. These tests were conducted with BP having thicknesses of 5.1 and 8.6 μm. Molecular diameters were obtained from the online database 3DMET. The inability of the liquids including water, which has a very small diameter, to flow through the BP is an indirect proof of its sub-nanometer pore sizes and high tortuousity arising from the extremely close knit SWNT networks. The significance of the results lie in the fact that the estimated pore sizes are of the same order as in molecular sieve membranes (0.28−0.7 nm) that are reportedly very effective in separation of gases.24,49 Current commercial gas separation techniques are mostly based on dense nonporous polymer membranes, in which transient pores come into existence through random thermal motion. Separation through these films occurs by a solution−diffusion mechanism.24 In a porous membrane, if the size of the pores is less than or comparable to the mean free path of a gas (generally of the order of 100 nm), then the diffusion rate of the gas molecules becomes inversely proportional to the square root of its molecular weight (Knudsen diffusion). This leads to a partial separation of a gas mixture. CNT-based filters that operate in this regime have been developed.13,23,49 For extremely small pores (